As Video editors go, Videoshop (free for a limited time, typically US$2.99) is first rate and easy to use. I prefer it to iMovie on iOS and just about anything I have tried.
You can start a new movie or load an old one. If it is one you have loaded you can add narration, music, sound effects, render the movie in slow or fast motion, and even add filters. There is a nice selection of titles, and you can add text to parts of the movie.
Using Videoshop is completely intuitive, with next steps being very obvious. Multiple videos can be combined, and there are controls for trims. There are also six included transitions to place between scenes. The output is up to 720P, and you can share your masterpiece via email, Vine, YouTube, Instagram or Facebook.
My testing revealed a very solid app. It was easy to go back and try things again if you don't like the first result. I found you can also create slideshows by loading still photos, then setting transitions and creating narration. The latest update also allows you to try your hand at stop-motion, but I didn't put that feature to the test.
Videoshop for iOS
Apple's iMovie ($4.99) is also powerful, and has a lot of pre-built templates for creating movies, but I liked the workflow in Videoshop much better. I could create a movie much more quickly, yet the output looked very polished.
The only thing I would improve is the text size if your add subtitles. The opening text is very nice, and allows for some attractive variations, but after that adding text is bare bones with no choice of text size, style or color.
Still, Videoshop is a big thumbs up. It won't be free for long, so if you are interested grab it ASAP. It's very well reviewed by users.
Videoshop is a universal app. It requires iOS 6.0 or later, and it is optimized for the iPhone 5.